I was diagnosed with clinical depression when I was 16 (along with bi-polar and borderline personality disorder) recently my boyfriend proposed to me and I accepted and now we are both busy telling our friends, family and planning a 2014 wedding.

My mother died when I was 13 from cancer and now I've become depressed and sad over the fact that she won't be at my wedding and that she will never meet her grandchildren when they are born, these feelings are destroying what is supposed to be a happy time in my life, I find myself in tears every single day about it.

Are my thoughts normal and what should I do to make myself happy again?

Yes this is normal. This is a little different then your story but when my can died i was completely thrown off track for a long time. A death like that can always take a while to heal from. A thing that worked for me was writing a letter to the person that has passed. It sounds kinda wierd even dumb (I was doubtful at first) but it worked really well, doing this helps you poor out all the emotions you have onto paper. And somehow this helps, not totally sure how yet but it worked. This may not work for you but its just a suggestion.
I believe you can get through this. Mainly because ive been through a lot in life and yes i may be young but i have a lot of insight into life and how things work.